It is known to deep-draw a thermoplastic foil to form a package part utilizing a mold having two mold halves which are brought together in a mold-closing position.
Two such foil webs of a single folded foil, of two separate foil strips, or of a flattened foil tube, may thus be introduced between a pair of mold halves, each of which is provided with a mold cavity and is constructed to allow the foil to be drawn and especially deep drawn into the respective cavity.
The two mold halves thus form a multipartite deep-drawing mold or form, each container part of which is formed in the respective mold half so that the two container halves are bonded together or fused between the mold halves of the closed form or mold.
When the two mold parts are separated, therefore, the finished synthetic resin container can be withdrawn.
Various mechanical devices or drives are used to bring the mold halves together and it is desirable to provide such drives so that they can operate at high speeds and also generate high mold-closure forces.
The principal mechanism used for this purpose is hydraulically actuated and the mechanism can include toggle linkages to generate the high closing forces and velocities, although directly operating multistep hydraulic cylinders can be employed as well. In smaller machines, however, indirect pneumatic drives and even electric-motor drives have been found to be advantageous.
In principle, however, all of these drives operate substantially similarly:
From a fully open position, with the mold halves at their maximum spacing, one or both of the mold halves can be displaced at high velocity to a stop position in which vacuum forming or deep drawing is effected, or into a first mold-closure position.
After a brief interval at this position, during which the deep-drawing operation can be effected, the mold is closed by further movement of one or both of the mold halves toward the other at a reduced velocity but with substantially higher force.
The drive is then shut down after mechanical locking of the form halves in the closed-mold position until forming of the article is complete. The forming of the article can include filling of a package defined by the deep-drawn foils.
After the end of the forming step, for removal of the completed article, the drive mechanism is operated to separate the mold halves at high velocity.
In the production of synthetic resin articles, especially hollow bodies of thermoplastic material as described, where two container halves are fused together at high pressure by movement of the mold halves together, there are differences in thickness of the foil between the mold halves of material-specific shrinkage processes which the foil must undergo.
This means, in systems in which the mold halves are located as described above for the prior art approach, that the closing and holding forces upon the mold do not remain constant, especially at the weld seams at which the synthetic resin is squeezed between the mold halves.
In many cases, therefore, because of the dimensional changes resulting from such shrinkage, the welding forces may be insufficient or irregular so that it is possible that excessively thin regions may be produced where the closing forces must be increased to compensate for dimensional changes, or sealing may be unsatisfactory where the force is insufficient because of such dimensional changes.